IGNEOUS ROCKS (Lat. igneus, fiery, from ignis, Skt. agoi. fire). Rocks produced as the re sult of solidification from a molten condition. Such rocks include lavas which have been poured out upon the earth's surface; the fillings of the fissures, pipes, and other passages in the earth's crust through which molten material was con ducted during its rise to or toward the surface: and the larger masses which consolidated at great depths. The agents of decomposition and disintegration of rock material and those of transportation and degradation necessarily ex pose to view these several types of igneous rocks, each of which possesses certain general and distin guishing characteristics. The most noteworthy general characteristic of the igneous rocks, when unmetamorphosed, is a massive structure with out lamination or bedding, such as is characteris tic of the sedimentary or elastic rocks on the one hand; or schistose structure, such as is developed in the metamorphic rocks. Inasmuch, however, as it has been shown that many metamorphic rocks (q.v.) have been formed from igneous rocks, it is clear that no sharp line can be drawn to separate these classes on the basis of structure, although in the main the above distinction ap plies.
Prominent among the textures characteristic of the igneous rocks are the granitic, the porphy ritic, and the vitreous or glassy. The granitic texture is crystalline throughout and consists of an interlocking mosaic of the different mineral constituents, the nearly uniform size of the grains indicating that the process of consolidation was essentially an uninterrupted one. and that prac tically the same conditions obtained during all stages of the process. In this mosaic the mineral constituents which first separated from the magma have more or less perfect crystal outlines, whereas those of later separation have irregular boundaries because sufficient space was not avail able in which to build out their crystal faces. The porphyritic texture, which is the next most common among the igneous rocks, has crystals of one or more of the mineral constituents of the rock imbedded in a base or ground mass of smaller crystals, or of rock glass. The crystals imbedded
in the ground mass arc known as porphyritic crystals or phenocrysts. It is generally supposed that the phenocrysts were crystallized out of the magma in a stage of the process of consolida tion earlier than that which produced the ground mass of the rock, and as most igneous rocks of porphyritic texture have been formed either at the surface of the earth or at quite moderate depths below it, it is supposed that the phenocrysts were formed at considerable depths before the magma rose to or near the surface. In the lavas that flow from Vesuvius. or from most other vol canoes, the phenocrysts may be picked out of the still molten lava as it flows from the volcano. The vitreous or glassy texture is one of com paratively infrequent occurrence, but is repre sented by obsidian (q.v.) and pitchstone. It is now generally recognized that many igneous rocks which have an entirely crystalline texture were once largely composed of rock glass, which has &vitrified through processes of chemical altera tion and crystallization.
In addition to the above textures characteristic of igneous rocks and dependent upon the state of crystallization of the rock substance, there are many others. One of the most common observed in lava (q.v.) is a peculiar crumpled lamination of the rock caused by the arrangement of mineral constituents of unequal dimensions with their longer axes parallel to the crumpled lines of the texture. This fluxion texture is conditioned by the flow of the lava, the crumpled lines indicating the direction of flow. Other textures, as the am•gdaloidal, seoriaeeous, and pumiceous, which indicate different grades of cellular or porous texture, are conditioned by the steam once held in the lava and the opportunities for expansion and escape of this steam as the lava approached the surface of the earth.